1. Field of the Invention
The present invention relates to methods and apparatus for treatment, handling, transport & storage of metallized iron ore produced by direct reduction techniques, and that products use in various types of iron and steelmaking furnaces.
2. History of the Invention
The annual world's direct reduction capacity by all processes in 1970 was less than three million tonnes, but in 1977 it had risen to 15 million tonnes. Over 30 million tonnes are indicated in 1980, and, by the end of the century with the world's steel capacity projected to be one billion tonnes, direct reduction will account for some 100 million tonnes. A current update with projections is presented by Miller, J. R., "Global Status of Direct Reduction--1977," Iron and Steel Engineer, September 1977.
Many reasons, both technical and economic, exist for this impressive growth, which largely is occurring in developing nations rather than in those already industrialized. The three most significant reasons for this growth are: (1) that direct reduction lends itself to limited production of metallized iron ore at moderate investments, in contrast to the massive capital outlays required for construction and operation of a modern blast furnace plant and related facilities such as those employed for coke production; (2) direct reduction can employ energy and fuels for ore reduction, such as natural gas, oil, and non-coking coals, but can be used, if at all, only in very limited quantities in the iron blast furnace process; and (3) melting and refining to steel or metallized iron ore produced by direct reduction techniques is readily accomplished in electric furnaces at moderate costs, in contrast to massive open-hearth or basic-oxygen steelmaking shops required for refining blast furnace molten pig iron or hot metal. Therefore, for a country having either or both iron ores and suitable fuel and energy supplies, direct reduction plants are ideal for providing that country with the basis for an iron and steel industry at minimal costs. In industrialized countries, while direct reduction plants have been constructed and are in operation, their growth has been hampered by fuel and energy shortages, particularly natural gas, from which reducing gases can be produced. However, as reserves of coal suitable for coke manufacturing for use in blast furnaces diminish, the development of coal gasification processes that utilize non-coking coals for producing reducing gases will provide incentives for further applications of direct reduction technology. Oil and natural gas rich nations that are deficient in iron ore are now construction direct reduction plants to metallize imported iron ore for domestic use and for export to world markets.
Whether for local use or export, a significant limitation on growth of direct reduction facilities results from the tendencies of metallized iron ore to reoxidize during short and long-term storage, transport, exposure to atmospheric oxygen, and weathering effects. Iron ore that reoxidizes, of course, loses its hard won degrees of metallization, and may spontaneously ignite and burn. Also, such metallized iron ore in storage in confined areas where it comes in contact with moisture can generate hydrogen. Thus, such decomposition of the metallized iron ore, in addition to losing the economic value thereof, can create serious hazards to personnel and equipent. The solution of such serious problems and hazards therefore would be a significant improvement in direct reduction technology and serve to promote additional applications and growth.
The products from the various direct reduction processes would also greatly add to the versatility of direct reduction and promote growth and so are also the subject of the present invention.
3. Prior Art
The accepted definition of direct reduction applies to processes in which reduction and resulting metallization of iron oxide ores occurs below the melting temperatures and the product is in a solid form. This characterizes direct reduction from other processes, such as a blast furnace, in which melting temperatures are attained and the products are molten slag and molten pig iron, usually called hot metal.
Although direct reduction is an old art, as stated hereinabove, it has played an insignificant role in the world's iron and steel industries until recently when a variety of excellent processes have been developed and commercialized. These processes can generally be classified into four types of reactor systems, with sub-classifications thereof based upon the required physical form of the iron ore feed, the type of reducing agent employed, and the physical form of the metallized product.
One such reactor system employs a moving-bed shaft furnace reactor that used iron ore agglomerates as the feed material formed from lump ore, pellets made from fine ore, or mixtures thereof. A preferred reducing agent for this system is gaseous mixtures consisting mostly of hydrogen (H.sub.2) and carbon monoxide (CO) that is commonly produced by steam reforming of natural gas. Metallized product from this system is as agglomerates excepting for a generally small proportion of fine material that may have been created wtihin the reactor. An example of one such system is described by Schroer, C. A., and Clark, D. W., "Operating a Midrex Direct Reduction Plant--Current State of the Art," Iron and Steel Engineer, August 1976.
Another system, a fixed-bed shaft furnace reactor also uses agglomerates as the feed material, with gaseous reducing agents, and the metallized product is agglomerates. An example of such system is described by Gearhart, H. E. and Jackson, K. A., "Production of Metallized Pellets by the HyL Process," Iron and Steel Engineer, March 1974.
A third of the four systems involves a rotary kiln reactor that also uses agglomerates as the ore feed material with the product therefrom appearing as metallized agglomerates. The reducing agent in this system is preferably solid carbonaceous materials, such as low-volatile and non-coking coals, and is often augmented by natural gas or fuel oil. Two examples of this system are shown in Reuter, G., "Proceso SL/RN," and Krebs, E., "Proceso Krupp," both in Reduccion Directa en America Latina published by ILAFA in 1974.
With each of these three reactor systems the metallized pellets or agglomerates produced therein have serious problems with reoxidation and product instability during storage and transport. To passivate the product it has heretofore involved additional and costly processing procedures and equipment, such as that illustrated by Pietsch, W., "Storage, Shipping and Handling of Midrex Iron," Preprint No. 76-B-317, SME-AIME Fall Meeting, Denver, Colo., Sept. 1-3, 1976, which passification problems can be, to a large extent, solved by the present invention at minimal cost.
A fourth system utilizes fluidized bed reactors that handle an iron ore feed of fine solids rather than as agglomerates. Gaseous reducing agents for this system are preferably produced by steam reforming of natural gas. The product from such system appears as metallized fines and, prior to the process of the present invention, other processes have generally practiced high-temperature briquetting of that product to convert it entirely into stabilized metallized agglomerates. An example of such high-temperature briquetting was presented by the present inventor, Davis, W. L., Jr., et al, in an article entitled, "Briquetacion del Mineral de Hierro de Alto Tenor de Reduccion Directa en Lechos Fluidizados," in Reduccion Directa en America Latina published by ILAFA in 1974.
Production of metallized iron ore within a fluidized bed reactor system has been described in detail in an earlier application for United States patent entitled, "Method and Apparatus for Producing Metallized Iron Ore," Ser. No. 899,318. Therein was discussed the stringent requirements on the ores, the expense and complexity of ore preparation plants, the limitations on incremental scaleup capacity of production plants, the thermodynamic and kinetic limitations on reductant gas utilizations and the like. Also, the problems of stabilizing the product produced by such reactor system were discussed briefly therein, and a method for post-treating that product was claimed. This process involves, in a fluidized bed apparatus, the steps of passing cold inert gases counter-currently through a moving bed of metallized iron ore, spraying water over the bed to reduce the temperature, and drying the cooled metallized iron ore. The present invention improves and elaborates upon this procedure to include providing solutions for problems involved in treating, handling, storage and transport of metallized iron ore, as well as problems of utilizing metallized iron ore in certain iron and steel-making furnaces and operations.
Detailed discussions of these problems are provided by Greenwalt, R. W., and Stephenson, J.G., "The Role of Agglomeration in Direct Reduction Processes," Agglomeraion 77, published by the AIME in 1977 (American Institute of Mining, Metalurgical, and Petroleum Engineers, Inc.) and by Rollinger, B., "Steel via Direct Reduction," published in the January 1975 issue of Iron and Steelmaker (an AIME publication.
Generally for reactor systems that produce metallized pellets or metallized agglomerates, with time the products tend to reoxidize and liberate heat which reoxidation and heat liberation is also dependent upon the conditions of the product, such as whether it is wet or dry, or the like. If stockpile quantities are large, as normally are desired in the iron and steel industry, heat accumulates, internal temperatures rise, and spontaneous ignition may occur and continuous burning takes place. Moreover, if such material in stockpile comes into contact with moisture, reoxidation accelerates and hydrogen gas is evolved which creates an extremely hazardous condition in confined areas.
Where metallized iron ore is produced as fine sizes or powders, having large surface areas, expensive and complex high-temperature briquetting operations followed by briquette cooling, have been employed to stabilize the product. Such operations have required complete blanketing and purging of the product with inert gases to exclude atmospheric oxygen during the briquette forming process. The objectives of such briquette forming are to decrease the surface areas of fine particles by producing dense briquettes to presumably decrease the tendency of the material to reoxidize or be pyrophoric at room temperatures, with the second objective being to make dense agglomerates that presumably are more amenable for use in electric furnaces. In meeting these objectives, those skilled in the art have established stringent requirements that essentially 100 percent of the powder must be converted to briquettes, with all individual briquettes being of extremely high density so as to be immune from pyrophoricity, and having high strengths so as to withstand and minimize degradation and the generation of fine particles during handling. Such requirements, of course, add greatly to the complexity and capital costs of briquetting facilities. For example, extremely high operating and repair and maintenance costs are incurred because of relatively short service lives of briquetting rolls, roll pocket insert materials, and check plates at high temperatures. High cost of operations are all incurred because of the complex equipment needed for hot screening, inert gas purging, and the recycling of hot unbriquetted material to the briquetting machines. In fact, it is not unusual to note a "doubling-up" of equipment to provide 100 percent backup, and despite such doubling-up, the reliability and on-line availability of such facilities leaves much to be desired. Finally, the resulting briquettes are by no means completely immune to reoxidation. Although they are considerably more stable in storage than porous metallized pellets, care must still be exercised to avoid "critical stockpile heights" as discussed in the cited reference by Greenwalt and Stephenson.
Such complex and expensive agglomeration facilities and such operating problems are unnecessary and are eliminated in the practice of the present invention. As is covered later herein, the assumptions that have led to the described complex and expensive procedures and equipment for the handling of metallized iron ore are, at best, shown to be only partly valid. The present invention furnishes new insights and interpretations of scientific and technical data whereby greatly improved and simplified methods and apparatus are provided that eliminate instability and reoxidation problems for all types of metallized iron ore products, thereby greatly enhancing the versatility for use of such products in various iron and steelmaking furnaces and operations.
Within the knowledge of the inventor there has not heretofore existed an arrangement of apparatus like that of the present invention, nor a process for its use like that of the present invention, and therefore the present invention is believed to be both novel and unique.